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46 P.S. Liu and D.M. Williams
Fig. 15 ( a ) Axial time-of-flight magnetic
resonance angiography ( MRA ) image in the
upper leg without any flow selection ( satura-
tion band ) demonstrates flow in both arterial
( arrow ) and venous ( arrowheads ) directions
with the superficial femoral arteries ( SFA) and
superficial femoral veins (SFV) , respectively.
No intravenous contrast was given. ( b ) Axial
time-of-flight MRA image in the upper leg
with inferior saturation band (arterial flow
selection) demonstrates near complete nullifi-
cation of venous signal and only flow-related
enhancement in the arterial structures. Arrows =
arterial flow in bilateral SFAs. ( c ) Axial time-
of-flight MRA image in the upper leg with
superior saturation band (venous flow selec-
tion) demonstrates reversal of panel b , with
near complete nullification of arterial signal
and only venous flow-related enhancement.
Arrowheads = venous flow in bilateral SFVs
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Chapter 3 Contemporary Imaging of Aortic Disease 47
to “prepare” the imaging slice, ultimately leading to nullification of blood
protons within the slice and yielding excellent depiction of intraluminal and
mural abnormalities.
22
Contrast-enhanced MRA has become the predominant
method of evaluating the vascular tree, as the injection of Gadolinium-based
contrast agents allows accurate depiction of the luminal appearance on fast
T1-weighted imaging
20,
23
(Fig. 12b and c ). Because MR techniques do not
have ionizing radiation, similar images at multiple time points can be acquired
without harm to the patient. Therefore, imaging techniques are frequently used
in MRA which will demonstrate multiple phases of vascular contrast, including
arterial, venous, and delayed phases of imaging. Contrast-enhanced MRA is
particularly useful for aneurysmal disease of the aorta and iliac arteries, as the
slow swirling flow in such aneurysms can degrade other MR sequences.
24
For
aneurysm evaluation, both arterial and delayed images are critical to evaluate
(Fig. 12b and c ); the arterial images will accurately demonstrate the luminal
contour while delayed images are useful for looking at the vascular wall and
extent of mural thrombus formation.
20
Prior to the use of contrast-enhanced MRA sequences, luminal flow informa-
tion could be obtained on MRI from noncontrast techniques. With recent news
that Gadolinium-based contrast agents may be linked to a skin and integument
disorder called nephrogenic systemic fibrosis in patients with impaired renal
function, there is a renewed interest in noncontrast MRA techniques.
25,
26
Principal
among these techniques is a variant T1-weighted gradient echo sequence called
“time-of-flight” (TOF) imaging. TOF MRA relies on the difference in longitu-
dinal magnetization of flowing protons in a bloodstream that enters into a slice
of interest versus static protons within that slice of interest.
27
This signal differ-
ence can be manipulated by imaging parameters to provide luminal contrast. In
fact, by using selective saturation bands adjacent to the slice of interest, isolated
arterial or venous flow can be displayed
22,
27
(Fig. 15a–c ). Other techniques such
as phase-contrast MRA and bright-blood steady-state free procession provide
high signal intensity within vascular lumen without the addition of intravenous
contrast
18,
20,
27
(Fig. 16 ). Given that many patients with aortic aneurysmal dis-
ease often have coexistent other medical comorbidities, including chronic renal
Fig. 16 Axial balanced steady-state free proces-
sion image through the chest demonstrates high
signal intensity within the aortic lumen ( arrows ).
No intravenous contrast was administered
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48 P.S. Liu and D.M. Williams
insufficiency, such imaging techniques may become increasingly valuable as an
alternative to both iodinated and Gadolinium-based contrast agents.
MRA has numerous benefits compared to other imaging modalities
including native/direct multiplanar imaging, high soft tissue contrast, flow
quantification, and the ability to evaluate both the vascular lumen and
its wall.
20
As opposed to the rotating machinery involved in CT, MRI local-
izes signal in the body via the sophisticated interplay of magnetic gradients
and radiofrequency pulses.
17
Therefore, it is not dependent on particular
geometric constraints for acquiring images, and can image the body in any
plane prescribed by the user. This is particularly useful in the thoracic aorta,
where images can be obtained in the oblique sagittal plane to anatomically
follow the oblique course of the thoracic aorta. The intrinsic difficulty in MRI
of visualizing calcification can actually be useful for MRA applications –
cortical bone structures are relatively inconspicuous on MRI versus CT where
such structures can be quite confounding due to their high attenuation
7,
20
(Fig. 17 ). With respect to CTA, there are some recognized disadvantages
of MRA that must be considered, including decreased spatial resolution,
increased examination length, contraindication with implantable electroni-
cally active hardware (such as pacemakers or defibrillator devices), interfer-
ence with imaging by ferromagnetic components of vascular prostheses, and
decreased equipment availability.
13
Additionally, the technical variability of
Fig. 17 Thick slab maximum intensity projection (MIP) from postgadolinium
fat- suppressed T1-weighted oblique sagittal spoiled gradient echo data set demon-
strates normal thoracic aorta. Note the lack of artifact related to osseous structures
incorporated into the MIP image, as cortical bone is dark on magnetic resonance
imaging ( MRI )
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Chapter 3 Contemporary Imaging of Aortic Disease 49
Fig. 18 ( a ) Axial balanced steady-state free procession image through the chest demonstrates a 4.2-cm thoracic
aortic aneurysm of the descending thoracic aorta. ( b ) Axial balanced steady-state free procession image through
a similar level 9 months later redemonstrates a 4.2-cm aortic aneurysm without interval change. This set of
noncontrast images highlights how magnetic resonance angiography ( MRA ) can excel in following known
aneurysmal disease without ionizing radiation or nephrotoxic iodinated contrast
MRA is higher than in CTA due to fluctuations in technologist skill level and
inconsistency in available pulse sequences or gating techniques on different
machines. Despite those limitations, MRA remains a useful vascular imaging
tool whose future utility may continue to increase, given the recent increased
concern about radiation doses in medical imaging and nephrotoxic effects of
iodinated contrast material.
28
MRA can be used effectively in the management of aneurysmal disease,
particularly in detection and surveillance. MRA is indicated as a screen-
ing technique in patients with high risk of developing aneurysms, such as
those with connective tissue disease or vasculitis.
13
A patient with known
aneurysmal disease is well suited for noncontrast MRA surveillance, given
that this technique has no ionizing radiation and does not use a nephrotoxic
contrast agent (Fig. 18a and b). MRA provides excellent detail about the
status of disease, including assessment of the true aneurysm diameter.
13,
22
MRI is not frequently used in the emergent evaluation of patients with
suspected aortic rupture due to examination time and generalized equip-
ment availability. Because of slice thickness considerations, MRA has not
been used as frequently as CTA for preoperative stent-graft measurements;
nevertheless, studies have demonstrated that high spatial-resolution MRA
can in fact be used as a sole preoperative test for endovascular planning,
including integration with automated vessel analysis programs.
29–
31
MRA
can also be used in the postoperative period to follow patients with both
open and endovascular repairs of aortic aneurysms. In patients with prior
endovascular repair of aneurysm, the ability to visualize the stent graft
itself is highly dependent on the graft material composition.
12
Nitinol-
based devices cause only minor distortions on acquired images, while
stainless steel devices create substantial artifact that often precludes any
detailed evaluation.
32
As opposed to CTA, the device itself is usually seen
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50 P.S. Liu and D.M. Williams
Fig. 19 ( a ) Axial fat-suppressed postcontrast T1-weig-
hted spoiled gradient echo image of the abdomen dem-
onstrates a 4.8-cm abdominal aortic aneurysm. There is
also an indwelling stent graft. Along the right posterior
aspect of the aorta, there is a small area of high signal
intensity ( arrow ), outside the stent graft itself, suspect
for an endoleak. ( b ) Reformatted maximum intensity
projection image using arterial data set from an axial fat-
suppressed postcontrast T1-weighted spoiled gradient
echo sequence, demonstrating an enlarged lumbar
artery leading into the aneurysm sac ( arrows ), thought
to be responsible for the type II endoleak
in negative contrast versus the blood pool on T1-weighted gradient echo
images. Endoleaks following endovascular repair with nonferromagnetic
materials are also well delineated by MRA, with published studies dem-
onstrating at least similar efficacy to CTA and perhaps superior detection
of subtle type II endoleaks
33–
35
(Fig. 19a and b ). Newer techniques in MRA
that allow blood pool and dynamic imaging may in fact improve endoleak
detection even further and is an area of active investigation.
36,
37
MRA is a
robust tool for imaging both preoperative and postoperative patients with
aneurysmal disease.
Catheter-Based Vascular Imaging
The technical improvements in CTA and MRA have almost completely elimi-
nated the role of catheter-based angiography from diagnostic imaging of the
aorta and its first-order branches. Limited roles for catheter-based angiography
using vascular contrast agents remain in fine-adjusting final endograft place-
ment with respect to renal or other critical branch artery origins, in monitoring
branch artery interventions such as embolization of renal or internal iliac arter-
ies, or in demonstrating and/or treating endoleaks. These are straightforward
and fairly routine, and will not be discussed further.
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Chapter 3 Contemporary Imaging of Aortic Disease 51
Renal impairment has always been a relative contraindication to the use of
iodinated contrast materials. The use of renal-protective strategies such as oral
N -acetylcysteine or intravenous bicarbonate infusions appear to mitigate the
renotoxicity of iodinated contrast agents, although unequivocal demonstration
of their benefit has not yet been documented. Nevertheless, the safety margin
of these agents is so broad that preimaging administration of one or both has
won wide acceptance. Gadolinium-based contrast agents, the mainstay of
contrast-enhanced MRI, were championed as renal-friendly contrast agents
used in off-label applications as an x-ray contrast material, until the discovery
of the nephrogenic systemic sclerosis specifically affecting patients in renal
failure. In contrast, the use of intravascular carbon dioxide contrast, long
championed by pioneers in the interventional radiology community, is gaining
wide acceptance by all peripheral interventionalists. Safe use requires unflag-
ging attention to numerous safety precautions and clinical rules of thumb,
which have to be adopted in toto by the entire angiographic team.
38
For example,
intra-arterial carbon dioxide cannot be considered safe if injected above the
diaphragm. This and other precautions are based on the physical properties
of carbon dioxide gas and idiosyncrasies of delivering a high-volume arterial
bolus in a clinically tolerable fashion. For example, a patient with a previ-
ous right radical nephrectomy needed endograft treatment of an abdominal
aortic aneurysm. This is an ideal application of carbon dioxide angiography.
The goal is to display the renal and internal iliac artery origins, in order to guide
endograft deployment; in this well-defined clinical scenario, carbon dioxide
can reliably substitute for iodinated contrast agents. Subtraction angiography
with carbon dioxide injections unequivocally localizes the solitary left renal
artery and internal iliac arteries (Fig. 20a and b ). The inferior mesenteric artery
(IMA) is also filled because of its anterior origin off the aneurysm sac and
preferential filling of nondependent structures by the buoyant gas (Fig. 20b ,
arrow). The IMA continues to fill after endograft deployment, both from iliac
artery injection and from suprarenal aortic injection (Fig. 20c–e ). This case
illustrates a limit in our current understanding of carbon dioxide angiography,
which has been encountered in the context of TIPS construction, diagnosis of
gastrointestinal or traumatic hemorrhage, or arteriovenous shunting, namely,
that carbon dioxide angiography may be too sensitive. The reliability of car-
bon dioxide in demonstrating clinically significant endoleaks requires further
systematic study.
IVUS imaging of the aorta is based on 8–9 Fr catheter systems; imaging of
arteries such as the superior mesenteric or renal arteries is best accomplished
with 5–6 Fr systems. The former allows visualization of a 60-mm-diameter
field of view, the latter an 8- to 20-mm-diameter field of view. IVUS is best
for high-resolution clarification of specific localized anatomical questions.
For example, in treating aortic aneurysmal disease, it is sometimes useful
to place an IVUS catheter at the celiac or left common carotid origin, to
allow absolute localization of the vessel origin independent of beam diver-
gence, center of the fluoroscopic field, magnification, or other confounding
geometrical distortion. IVUS imaging is even more generally useful during
interventions to treat aortic dissection. IVUS allows unequivocal distinction
between true and false lumens at any given level and permits immediate
localization of a catheter or guidewire with respect to the aortic true and
false lumens, without contrast injection. As such, it can guide interventions
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52 P.S. Liu and D.M. Williams
Fig. 20 Five images from carbon dioxide angiography
during deployment of a Zenith endograft in a patient
with prior right nephrectomy. Initial bolus injection
( a ) localizes the left renal artery origin, confirmed
after deployment of the first two covered segments ( b ).
Note expected filling of the patent inferior mesenteric
artery ( IMA ) ( b , arrow ). After deployment of the iliac
limbs, carbon dioxide gas injected into the right iliac
artery ( c ) continues to fill the IMA, indicating a subtle
junctional endoleak. It would be unusual to see such a
leak after injection of iodinated contrast material. Final
aortography with carbon dioxide confirms sparing of
the left renal and internal iliac arteries ( d ). However,
there is continued filling of the IMA ( e ), but this find-
ing is inconclusive because, due to its low viscosity,
the carbon dioxide quickly fills the superior mesenteric
artery, middle colic artery, and arc of Riolan ( e , arrow-
heads ). Further research is needed to determine the
clinical significance of endoleaks demonstrated by
carbon dioxide angiography
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Chapter 3 Contemporary Imaging of Aortic Disease 53
in the dissected aorta as well as the superior mesenteric or other branch
artery. Passing the IVUS over a guidewire allows positive identification
of the entire course of the wire, which can inconspicuously traverse from
one lumen across the flap into another. Lack of appreciation of an errant
guidewire pathway, when implanting an aortic stent or endograft, can lead to
anatomical blunders which can be disastrous at worst and time-consuming
to correct at best. IVUS may be used to guide creation of fenestration tears
and assess the response of the true lumen to creation of a fenestration tear
or implantation of an endograft IVUS catheters. The guidewire lumen of the
IVUS catheter can be attached to an electronic manometer for direct correla-
tion of pressure drops with anatomical features of the aortic lumen. These
and other applications make IVUS an indispensable tool in any high-volume
aortic practice.
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Chapter 3 Contemporary Imaging of Aortic Disease 55
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